The respiratory system constitutes a vital function of most animals, including man. Therefore, it is an important objective in health care to control this function. This includes assessment of the respiratory rhythm and intensity. Normally, in patients breathing spontaneously, this assessment is performed by human observation and a stop-watch, using three of the natural senses: hearing (the respiratory sounds), vision (abdominal movements), and tactile sensing (the flow of warm, and moist air out of the mouth and nose of the subject).
Human observation is not practical in all situations, for example, due to limited access to the subject, or limited availability of personnel. Life-threatening respiratory depression and/or apnea can occur suddenly, after long periods of normal respiratory function. Complications can be either obstructive, i.e., caused by partial or complete blockage of the airways, or due to insufficiencies in the central respiratory control system, or a combination of both. The sleep apnea syndrome is an example of the former, whereas the latter can occur after use of morphine or other pain-relieving substances. The etiology of the sudden infant death syndrome, striking mostly newborns, is still controversial, but seems to be related to insufficient respiratory control.
Consequently, there is a need for automated respiratory monitoring of these and other groups of patients. This need is well recognized; and a number of devices have been reported. The simplest approach is to detect the abdominal movements, by impedance measurement, pressure variations in a mattress, or by measuring the circumference of belts or straps placed around the chest. These detectors, however, have the serious error of dubious response to obstructive respiratory disorder. If an obstructive apnea occurs, the subject is usually still making efforts to breathe, and the movement detectors have difficulties in discriminating between a successful effort or one that is not.
A more successful approach is to detect the flow of air at the mouth and nose region of a subject. This can be done using small thermistors, or by measuring small pressure variations, or acoustic noise generated by the flow. However, placing sensors in this region is not comfortable for the patient, and involuntary displacement of them may cause false alarms, since this condition is normally misinterpreted as an apnea.